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Post Calibration Inspection (Automatic Exposure Control)
About The purpose of Automated Exposure Control (AEC) is to deliver consistent, reproducible exposures across a wide range of anatomical thicknesses, tube potentials, and patients. „Detectors used in AEC cells include fluorescent screens with PMTs/photo diodes, ionization chambers, and solid state detectors. History The history of AEC comes from the old films days of radiography and using Optical Density (OD) to perform routine AEC testing. OD uses the films contrast (gray scale) printed by a sensitometer and then read by a sensitometer.. Today, using Conventional Radiography (CR) and Digital Radiography (DR) the use of film and wet processing devices have become obsolete. New CR/DR modalities are no longer are bound by contrast limited calibrations but are now measured and limited by noise (pixels and EI values). Jones, Kyle. ""Calibrating Automatic Exposure Control for Digital Radiography". UTMD Anderson Cancer Center, Houston, TX. 2/13/2016. AEC Thickness AEC thickness (patient thickness) automatically adjusts exposure time and milliampere (mAs) based on the size or anatomical thickness of the patient. This involves beam quality of the detector cells to absorb the correct amount of energy, sensitivity of the AEC cells, x-ray scatter absorption, and the contrast factor of the grid. AEC Thickness tracking can be performed using a range of thicknesses from 2cm to 6 cm. In addition, a standard chest phantom could be used to simulate thickness. Calculate Signal-To-Noise (SNR) values for each thickness settings. The SNR values should be greater than 40 with a maximum deviation less than 15%. Test Frequency Semi-Annually SID #1. Table – 40" #2. Chest/Wall – 72" Technique Factors 50kVp, 60kVp, 70kVp, 90kVp, 120kVp, 20cm (chest) acryllic, no filtration (Air Kerma) Test Tools A X-ray Quality Assurance meter (e.g. RTIs Piranha or Barracuda), 14x17 cassette Procedures TABLE 1. Place a QA meter on top of table. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock table in place. 3. Insert 14 x 17 cassette into table Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. CHEST/WALL 1. Place a QA meter at chest/wall Bucky. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock chest Bucky in place. 3. Insert 14 x 17 cassette into chest Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. Sensor selector/location Sensor selector/location comes from the word 'sensor' that is widely used in the description of measurement systems. Any AEC sensor is based on a simple concept that physical location of an AEC sensor must be excited by an external stimulus (x-rays) to produce an electric signal. Test Frequency Semi-Annually Procedures TABLE 1. Inspect AEC cell sensors on top of table. 2. Record pass of fail results. CHEST/WALL 1. Inspect AEC cell sensors at the chest/wall Bucky. 2. Record pass of fail results. Reproducibility „Reproducibility is the Air Kerma ability to deliver a consistent, repeated, and measurable (mR) exposure to the AEC cell. The Coefficient Of Variance (COV) should be less than 0.05 (AAPM 14). In addition, the COV for a mean pixel value, mAs, and entrance exposure for a series of exposures is less than 5%, and the SNR should not vary by more than 15% from Mean SNR value. Test Frequency Semi-Annually SID #1. Table – 40" #2. Chest/Wall – 72" Technique Factors 90kVp, 1 sec, no filtration (Air Kerma) Test Tools A X-ray Quality Assurance meter (e.g. RTIs Piranha or Barracuda), 14x17 cassette Procedures 1. Place a QA meter on top of table. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock table in place. 3. Insert 14 x 17 cassette into table Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. Density correction „ Density correction adjusts mAs upward or downward in increments of 25 to 30% per mAs step. „ 0.15 to 0.30 OD per step (AAPM 74). 4-step controller should adjust mAs by 20 to 25% per step (AAPM 14). Test Frequency Semi-Annually SID #1. Table – 40" #2. Chest/Wall – 72" Technique Factors -4, -2, -1, 0 (N), 1, 2, 4, 80kVp, no filtration (Air Kerma) Test Tools A X-ray Quality Assurance meter (e.g. RTIs Piranha or Barracuda), 14x17 cassette Procedures TABLE 1. Place a QA meter on top of table. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock table in place. 3. Insert 14 x 17 cassette into table Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. CHEST/WALL 1. Place a QA meter at chest/wall Bucky. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock chest Bucky in place. 3. Insert 14 x 17 cassette into chest Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. AEC balance AEC balance is used to achieve a single or combination of consistent AEC cell exposures. Other manufacturers still have tunable AEC cells: „ #1. Pots in generator;„ #2. Software interface;„ #3. Pots in detector housing± 5% across all combinations (AAPM 14). Testing center ©, left (L), and right ® as well as all three (C + L+ R) cell combinations. Test Frequency Semi-Annually SID #1. Table – 40" #2. Chest/Wall – 72" Technique Factors no filtration (Air Kerma) Test Tools A X-ray Quality Assurance meter (e.g. RTIs Piranha or Barracuda), 14x17 cassette Procedures TABLE 1. Place a QA meter on top of table. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock table in place. 3. Insert 14 x 17 cassette into table Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. CHEST/WALL 1. Place a QA meter at chest/wall Bucky. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock chest Bucky in place. 3. Insert 14 x 17 cassette into chest Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. kVp Tracking kVp tracking is an accuracy test and tracking portion of the AEC cells, only the techniques and/or values used clinically by the radiologist or facility need to be tested, up to a maximum of 3 values. If only 1 or 2 kVp’s are used, only those kVp’s need to be tested. If three or more kVp's are used, only three of those used need to be tested. Test Frequency Semi-Annually SID #1. Table – 40" #2. Chest/Wall – 72" Technique Factors no filtration (Air Kerma) Test Tools A X-ray Quality Assurance meter (e.g. RTIs Piranha or Barracuda), 14x17 cassette Procedures TABLE 1. Place a QA meter on top of table. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock table in place. 3. Insert 14 x 17 cassette into table Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. CHEST/WALL 1. Place a QA meter at chest/wall Bucky. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock chest Bucky in place. 3. Insert 14 x 17 cassette into chest Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. Backup timer Backup timer is the exposure termination after a certain period of time or delivered mAs.„ 600 mAs 600 mAs ≥ 51 kVp;„ 2,000 mAs < 51 kVp. The backup timer is designed to prevent catastrophic damage to x-ray tube by terminating exposure after a maximum time or when the AEC fails. A typical timer setting is 5,000 milliseconds or x-rays are stopped after a 5 seconds. Test Frequency Semi-Annually SID #1. Table – 40" #2. Chest/Wall – 72" Technique Factors no filtration (Air Kerma) Test Tools A X-ray Quality Assurance meter (e.g. RTIs Piranha or Barracuda), 14x17 cassette Procedures TABLE 1. Place a lead sheet over AEC cells so they are completed shielded on top of table. 2. Activate only the center cell and close collimator blades 3. Set photo timer mode to 65kVp. 3. Make an exposure until AEC backup timer shuts off X-ray beam. 4. Measure mAs exposure and record. 5. A visual signal and reset should be triggered prior to the accumulation of 600mAs CHEST/WALL 1. Place a QA meter at chest/wall Bucky. 2. Set up a 20cm acryllic to simulate a patient chest. 3. Set the collimator to 40" and lock chest Bucky in place. 3. Insert 14 x 17 cassette into chest Bucky. 4. Take a series of shots using the above technique without any filtration. 5. Measure mAs exposure and record. References Category:Medical imaging